It has been known to use both centrifugal and axial flow compressors in the past, however most present gas turbine engines are provided with axial flow compressors. While it is well known that centrifugal compressors are more robust, and more easily manufactured than axial flow compressors, axial flow compressors have the ability to consume far more air than a centrifugal compressor having the same frontal area. The axial flow compressor can also be designed for higher pressure ratios than the centrifugal compressor. Since the airflow is an important factor determining the amount of thrust a gas turbine engine produces, this means the axial flow compressor will give more thrust than the centrifugal compressor for the same frontal area, hence it is the more obvious choice for present day gas turbine engines.
An axial flow compressor comprises one or more rotor assemblies that carry blades of aerofoil section, the rotors being mounted between bearings. The rotor assemblies are carried within a casting within which are located stator blades. The compressor is a multi-stage unit as the amount of work done (pressure increase) by each stage is small, a stage consists of a row of rotating blades followed by a row of stator blades. The reason for the small pressure increase across each stage is that the rate of diffusion and the deflection angle of the blades must be limited if losses due to air breakaway at the blades, and subsequent blade stall are to be avoided.
The condition known as stall or surge occurs when the smooth flow of air through the compressor is disturbed. Although the two terms "stall" and "surge" are often used synonomously there is a difference which is mainly a matter of degree. A stall may affect only one stage or even a group of stages but a compressor surge generally refers to a complete flow breakdown through the compressor.
The value of airflow and pressure ratio at which a surge occurs is termed the "surge point". A compressor must obviously be designed to have a safety margin between the airflow and compression ratio at which it will normally be operated and the airflow and compression ratio at which a surge will occur.